1. Field of the Invention
The present invention relates to a vacuum processing system, and more particularly to a vacuum processing system where several vacuum process steps are applied to substrates such as wafers by the use of a vacuum container for storing and transporting the substrates under vacuum.
2. Description of the Prior Art
In integrated-circuit manufacturing, to prevent oxidation of wafers, there has been known a vacuum processing system which employs a vacuum container for transporting the wafers from a process station to a next process station, or storing the wafers during waiting process steps at a process station. One conventional vacuum container called a vacuum carrier is disclosed in Japanese Laid-Open patent publication 62-181441. The vacuum carrier comprises a carrier body and a carrier cover which connect each other with air-tight seal by differential pressure. The carrier body has a plurality of slits in which the wafers are placed.
The carrier body and the carrier cover are held closed by differential pressure through a vacuum seal such as an O-ring interposed between the carrier body and the carrier cover, whereby the interior of the vacuum carrier is held in vacuum.
In order to place the wafers into the vacuum carrier, first the vacuum carrier is placed in an upper load lock, then a lid of the upper load lock is closed, and the interior of the upper load lock is evacuated by a vacuum pump. Thereafter only the carrier body is lowered into a lower load lock in vacuum by lowering a stage connected to an elevator while the carrier cover remains in the upper load lock, and the wafers are transferred from a vacuum process chamber to the carrier body by a transferring device such as a robot. After transfer of the wafers into the carrier body, the carrier body is raised and joined to the carrier cover. After the pressure in the upper load lock is raised to ambient, the vacuum carrier is separated from the upper load lock and transported to a next process station.
In order to transfer the wafers in the vacuum carrier to the next process station, first the vacuum carrier is placed in the upper load lock, then the lid of the upper load lock is closed, and the interior of the upper load lock is evacuated by the vacuum pump. Only the carrier body is then lowered into the lower load lock by lowering the stage while the carrier cover remains in the upper load lock. Thereafter, the wafers are transferred from the carrier body to the vacuum process chamber by the transferring device.
By repeating the above steps, the wafers are transported from a process station to a next process station while the wafers are kept under vacuum.
However in the conventional vacuum processing system, the transportable vacuum carrier is not provided with a vacuum pump, the pressure in the vacuum carrier is raised with the lapse of time due to gas desorption from the inner surface of the vacuum carrier or leakage of air entering the vacuum carrier through the vacuum seal.
When the wafers are transported from a process station to a next process station, the wafers cannot always be transferred immediately after the transportation because the process station is occupied by preceding wafers. Thus the wafers must be waiting for the process. During waiting, the pressure in the vacuum carrier is raised, and native oxide is thus generated on the wafers in the vacuum carrier.